The effect is so impressive that patients may some day be able to skip chemotherapy but that will take more testing.

"This study has redefined the standard of care," Dr Powell said, who presented results from the large, three-year study at a meeting of the American Society of Clinical Oncology.

"The people who took arsenic lived longer," Dr. Nancy Davidson, president-elect of the American Society of Clinical Oncology, told Reuters.

The drug, arsenic trioxide, is approved for people with acute promyelocytic leukemia (APL) whose disease has returned.

Standard treatment for APL - a form of acute myeloid leukemia that strikes 1,500 people a year in the United States, involves chemotherapy and a form of vitamin A called all-trans retinoic acid, which helps 70 to 80 per cent of patients gain long-term remission.

However, about 25 per cent of those patients relapse and no longer respond to treatment.

Those patients often get arsenic trioxide.

But in a study sponsored by the National Cancer Institute in the US, Dr Powell and colleagues paired arsenic with standard treatment in newly diagnosed patients.

They found that 81 of 261 patients in the arsenic group were free of disease after three years, compared with 66 of 257 patients in the group who got the standard regimen alone.

"Among those who actually got arsenic, only five patients, or 2 per cent, relapsed," Dr Powell said.

"This is very impressive."

Doctors at the meeting believe the drug will be used as a first treatment right after chemotherapy.

"This really brings what has been a salvage treatment up front and actually improves survival," Dr Joseph Moore of Duke University Medical Centre said.

"The next question is how far do we move it forward?" Dr Powell said.

He said future studies would test whether patients can skip chemotherapy and just take arsenic.

Dr Mitchell Smith, director of the lymphoma service at Fox Chase Cancer Centre in Philadelphia says the therapy's effect was compelling but he was not ready to rule out chemotherapy, at least not without more study.

While most chemo treatments are toxic and work by killing cells, arsenic trioxide zeros in on disease-causing cells.

"It seems to kill preferentially," Dr Smith said.

Arsenic has been used as a traditional therapy in China for more than 2,000 years, but its use in the United States is still rather novel.

"Patients look up and pay attention when you mention you are going to treat them with arsenic," Dr Powell said.